Verify remote translog size-based flushing

[shourya035]

Description

Adds remote-store integration coverage for both sides of the opt-in behavior.

• Verifies the default-disabled path preserves the legacy moving-retention-boundary behavior without a periodic flush.
• Verifies enabled byte tracking accumulates bytes across refreshes and triggers a periodic flush.
• Verifies a successful commit clears the size trigger.
• Documents each integration scenario with point-by-point Javadocs.

Related issue

Tracks opensearch-project#22802.

Stack

1. Track translog bytes across commits #1 Byte tracking abstraction
2. Restore size-based remote translog flushes #2 Remote translog flow and opt-in setting
3. Verify remote translog size-based flushing #3 Remote-store integration coverage (this PR)

Depends on #2.

Testing

• ./gradlew :server:internalClusterTest --tests "org.opensearch.remotestore.RemoteStoreIT.testRemoteTranslogSizeFlushUsesLegacyRetentionBoundaryWhenDisabled" --tests "org.opensearch.remotestore.RemoteStoreIT.testRemoteTranslogSizeFlushTracksBytesAcrossRefreshes"
• ./gradlew :test:framework:precommit

Checklist

• Functionality includes integration tests.
• Commits are signed off.